This invention generally relates to plastic containers. More specifically, this invention relates to base portions of plastic containers for receiving a commodity and retaining the commodity during high-temperature pasteurization and during subsequent cooling, shipment, and use of the plastic containers.
Recently, manufacturers of polyethylene terephthalate (PET) containers have begun to supply plastic containers for commodities that were previously packaged in glass containers. The manufacturers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable, and manufacturable in large quantities. Manufacturers currently supply PET containers for various liquid commodities, such as juices. They also desire to supply PET containers for solid commodities, such as pickles. Many solid commodities, however, require pasteurization or retort, which presents an enormous challenge for manufactures of PET containers.
Pasteurization and retort are both methods for sterilizing the contents of a container after it has been filled. Both processes include the heating of the contents of the container to a specified temperature, usually above 70xc2x0 C., for a duration of a specified length. Retort differs from pasteurization in that it also applies overpressure to the container. This overpressure is necessary because a hot water bath is often used and the overpressure keeps the water in liquid form above its boiling point temperature. These processes present technical challenges for manufactures of PET containers, since new pasteurizable and retortable PET containers for these commodities will have to perform above and beyond the current capabilities of conventional heat set containers. Quite simply, the PET containers of the current techniques in the art cannot be produced in an economical manner such that they maintain their material integrity during the thermal processing of pasteurization and retort.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity is related to the percentage of the PET container in crystalline form, also known as the xe2x80x9ccrystallinityxe2x80x9d of the PET container. Crystallinity is characterized as a volume fraction by the equation:       %    ⁢          xe2x80x83        ⁢    Crystallinity    =                    ρ        -                  ρ          α                                      ρ          c                -                  ρ          α                      xc3x97    100  
where xcfx81 is the density of the PET material; xcfx81a is the density of pure amorphous PET material (1.333 g/cc); and xcfx81c is the density of pure crystalline material (1.455 g/cc). The crystallinity of a PET container can be increased by mechanical processing and by thermal processing.
Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET container along a longitudinal axis and expanding the PET container along a transverse axis. The combination promotes biaxial orientation. Manufacturers of PET bottles currently use mechanical processing to produce PET bottles having roughly 20% crystallinity (average sidewall crystallinity).
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. Used by itself on amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque (and generally undesirable as the sidewall of the container). Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a heated blow mold, at a temperature of 120-130xc2x0 C., and holding the blown container for about 3 seconds. Manufacturers of PET juice bottles, which must be hot filled at about 85xc2x0 C., currently use heat setting to produce PET juice bottles having a range of up to 25-30% crystallinity. Although these hot fill PET containers exhibit a significant improvement over the non-hot fill PET containers, they cannot maintain their material integrity during the thermal processing of pasteurization and retort, especially in their base portion, which, until now, have exhibited a roll-out failure.
Thus, the manufacturers of PET containers desire a container design that maintains its material integrity during subsequent pasteurization or retort of the contents within the PET container, and during subsequent cooling, shipment, and use of the PET containers. It is therefore an object of this invention to provide such a PET container that overcomes the problems and disadvantages of the conventional techniques in the art.
Accordingly, this invention provides for a plastic container having a particular base portion that allows the PET container to maintain its material integrity during subsequent mild pressures (35 to 175 kPa) encountered during high-temperature pasteurization or retort of the contents within the PET container, and during subsequent cooling, shipment, and use of the PET container. As used herein, xe2x80x9chigh-temperaturexe2x80x9d pasteurization and retort are pasteurization and retort processes in which the plastic container is exposed to temperatures greater than about 80xc2x0 C.
At its broadest, the invention is a plastic container for receiving a commodity and retaining the commodity during high-temperature pasteurization and subsequent cooling that includes an upper portion, a sidewall portion, and a base portion. The upper portion defines an aperture and is sealable with a closure. The sidewall portion, which defines a sidewall diameter, is connected to and extends generally downward from the upper portion. The base portion has a chime section connected to and extending generally downward and inward from the sidewall portion, and a push-up section connected to and extending generally upward and inward from the chime section to close the plastic container. The push-up section defines a push-up diameter, and the ratio of the sidewall diameter to the push-up diameter is at least 1.3:1.0.
Further features and advantages of the invention will become apparent from the following discussion and accompanying drawings.